Resumen

In classical P-Cygni profiles, theory predicts emission to peak at zero rest velocity. However,
supernova spectra exhibit emission that is generally blueshifted. While this characteristic
has been reported in many SNe, it is rarely discussed in any detail. Here, we present an
analysis of Hα emission peaks using a data set of 95 Type II supernovae, quantifying their
strength and time evolution. Using a post-explosion time of 30 d, we observe a systematic
blueshift of Hα emission, with a mean value of −2000 km s−1. This offset is greatest at early
times but vanishes as supernovae become nebular. Simulations of Dessart et al. match the
observed behaviour, reproducing both its strength and evolution in time. Such blueshifts are a
fundamental feature of supernova spectra as they are intimately tied to the density distribution
of ejecta, which falls more rapidly than in stellar winds. This steeper density structure causes
line emission/absorption to be much more confined; it also exacerbates the occultation of the
receding part of the ejecta, biasing line emission to the blue for a distant observer. We conclude
that blueshifted emission-peak offsets of several thousand km s−1 are a generic property of
observations, confirmed by models, of photospheric-phase Type II supernovae.